It is important to remark that a proper treatment of the boundary conditions is imperative due to its effect on the approximate solution. In order to avoid oscillations along the boundary, instead of using a least squares approximation as in the previous case studies, we have considered quasi-interpolation. More precisely, in order to exploit the shape-preserving property of the (T)B-spline representation, following the approach in [1]}, we approximate the boundary function through the Schoenberg operator (see [2]}, [3]}) along each edge. In other words, we consider the linear combination of the boundary TB-splines whose coefficients are obtained by evaluating the boundary function at the corresponding Greville abscissae (under the assumptions that linear polynomials belong to the ECT-space of interest).
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